The cartridges of conventional ball point pens are open to permit ink to be fed to the point. The secret behind the Fisher Space Pen lies in the unique design characteristics of the ink and the high precision manufacturing tolerances of the ball point and socket. The ink is fed to the ball point by gas pressure permitting the pen to write in any position. The cartridge is pressurized with nitrogen so that it doesn't rely on gravity to make it work. It is dependable in freezing cold and desert heat. It can also write underwater and upside down. Fisher's development of this pen couldn't have come at a more opportune time. The space race was on, and the astronauts involved in the Mercury and Gemini missions had been using pencils to take notes in space since standard ball points did not work in zero gravity. The Fisher cartridge did work in the weightlessness of outer space and the astronauts, beginning with the October, 1968 Apollo 7 mission began using the Fisher AG-7 Space Pen and cartridge developed in 1966.
